1. Field of the Invention
This invention relates to methods and apparatus for the medical treatment of disease of the heart. More particularly, this invention relates to a method and apparatus for treating cardiac arrhythmias by ablating in a vicinity of pulmonary venous tissue.
2. Description of the Related Art
Tissue ablation from the inner walls of hollow viscera of the body generally, and the vascular system in particular, has been found to be useful in the treatment of various medical conditions. Technological developments in intravascular catheters, manipulative instruments adapted to intravascular catheters, and catheter localization techniques have especially benefited the field of cardiology. Percutaneous transcatheter ablation has been used successfully in the treatment of conduction defects and arrhythmias of various types. Today, atrial tachyarrhythmias are a common application for ablative therapy.
Various ablative modalities have been employed in the past, such as ablation by direct heating. Energy can be conducted to the target tissue using various modalities, such as ultrasound, laser, resistive heating, and radiofrequency energy.
One ablative approach is the so-called “maze” technique. In general, the maze procedure attempts to block abnormal conduction patterns in the left atrium by establishing a maze-like pattern of linear lesions in the left atrial wall.
Atrial arrhythmias are known to be associated with abnormal electrical activity of tissue foci in the vicinity of the pulmonary veins, especially the superior pulmonary veins. Various ablative treatments of such foci have been attempted. For example, the production of linear atrial lesions by radiofrequency ablation, in combination with ablation of suspected arrhythmogenic foci has been performed using transcatheter techniques.
More recently, circumferential lesions at or near the ostia of the pulmonary veins have been created to treat atrial arrhythmias. U.S. Pat. Nos. 6,012,457 and 6,024,740, both to Lesh, disclose a radially expandable ablation device, which includes a radiofrequency electrode. Using this device, it is proposed to deliver radiofrequency energy to the pulmonary veins in order to establish a circumferential conduction block, thereby electrically isolating the pulmonary veins from the left atrium.
Radiofrequency ablation using multiple contiguous circumferential points, guided by electro-anatomical mapping is proposed in the document, Circumferential Radiofrequency Ablation of Pulmonary Vein Ostia: A New Anatomic Approach for Curing Atrial Fibrillation, Pappone C, Rosanio S, Oreto G, Tocchi M, Gugliotta F, Vicedomini G, Salvati A, Dicandia C, Mazzone P, Santinelli V, Gulletta S, Chierchia S, Circulation 102:2619–2628 (2000). It is emphasized that particular care must be exercised to ensure that the ablation sites are indeed contiguous; otherwise irregular electrical activity in the pulmonary vein may continue to contribute to atrial arrhythmia.
It has also been proposed to produce circumferential ablative lesions using ultrasound energy delivered via a cylindrical ultrasound transducer through a saline-filled balloon. This technique is described in the document, First Human Experience With Pulmonary Vein Isolation Using a Through - the - Balloon Circumferential Ultrasound Ablation System for Recurrent Atrial Fibrillation, Natale A, Pisano E, Shewchik J, Bash D, Fanelli R, M D; Potenza D; Santarelli P; Schweikert R; White R; Saliba W; Kanagaratnam L; Tchou P; Lesh M, Circulation 102:1879–1882 (2000). Ablation times on the order of 2 minutes are reported.
U.S. Pat. No. 6,117,101 to Diederich et al. discloses a technique for producing circumferential lesions for electrical isolation of the pulmonary veins. Using a balloon catheter, a cylindrical ultrasound transducer is provided on an inner member within a balloon, and emits a radial ultrasound signal that is sonically coupled to the balloon's outer skin.
A known drawback in the use of ultrasound energy for cardiac tissue ablation is the difficulty in controlling the local heating of tissue. There are tradeoffs between the clinical desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the ultrasound device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure.
In consideration of these, and other factors, it is appropriate, in designing a practical ultrasound emitter, to consider the amplitude of the ultrasound signal, the amount of time required for the energy application, the size of the electrode, and the contact area, as well as ease of positioning, withdrawal, and repositioning of the device so as to be able to conveniently produce multiple lesions during the same medical procedure.
Previous approaches to controlling local heating include the inclusion of thermocouples within the electrode and feedback control, signal modulation, local cooling of the catheter tip, and fluid assisted techniques, for example perfusion of the target tissue during the energy application, using chilled fluids. Typical of the last approach is described by Mulier et al. in U.S. Pat. No. 5,807,395.
Publications which describe various medical techniques of interest include:
Scheinman M M, Morady F. Nonpharmacological Approaches to Atrial Fibrillation. Circulation 2001;103:2120–2125.
Wang P J, Homoud M K, Link M S, Estes III N A. Alternate Energy Sources for Catheter Ablation. Curr Cardiol Rep 1999 July; 1(2):165–171.
Fried N M, Lardo A C, Berger R D, Calkins H, Halperin H R. Linear Lesions in Myocardium Created By Nd:YAG Laser Using Diffusing Optical Fibers: In Vitro and In Vivo Results. Lasers Surg Med 2000;27(4):295–304.
Eigler N L, Khorsandi M J, Forrester J S, Fishbein M C, Litvack F. Implantation and Recovery of Temporary Metallic Stents in Canine Coronary Arteries. J Am Coll Cardiol 1993; 22(4):1207–1213.
Synthetic Biodegradable Polymers as Medical Devices; by John C. Middleton and Arthur J. Tipton. 1998.
Keane D, Ruskin J, Linear Atrial Ablation With A Diode Laser And Fiber Optic Catheter. Circulation 1999; 100:e59–e60.
Ware D, et al., Slow intramural heating with diffused laser light: A unique method for deep myocardial coagulation. Circulation; Mar. 30, 1999; pp. 1630–1636.
Other medical technologies of interest are described in U.S. Pat. No. 5,891,134 to Goble et al., U.S. Pat. No. 5,433,708 to Nichols et al., U.S. Pat. No. 4,979,948 to Geddes et al., U.S. Pat. No. 6,004,269 to Crowley et al., U.S. Pat. No. 5,366,490 to Edwards et al., U.S. Pat. Nos. 5,971,983, 6,164,283, and U.S. Pat. No. 6,245,064 to Lesh, U.S. Pat. No. 6,190,382 to Ormsby et al., U.S. Pat. Nos. 6,251,109 and 6,090,084 to Hassett et al., U.S. Pat. No. 5,938,600 to Swartz et al., and U.S. Pat. No. 6,064,902 to Haissaguerre et al.
All of the patents and publications cited in this disclosure are incorporated herein by reference.